1. Field of the Invention
This invention relates to semiconductor device packages. More particularly, this invention relates to thermally enhanced, molded plastic semiconductor device packages.
2. Description of the Related Art
Molded plastic packages provide environmental protection to integrated circuit devices (dies). Such packages typically include at least one semiconductor device (die) having its input/output (I/O) pads electrically connected to a lead frame type substrate or an interposer type substrate, with a molding compound coating the die and at least a portion of the substrate. Typically, the I/O pads on the die are electrically connected to bond sites on the substrate using either a wire bonding, tape bonding, or flip-chip bonding method. The lead frame or interposer substrate transmits electrical signals between the I/O pads and an electrical circuit external to the package.
In semiconductor device packages having a lead frame type substrate, electrical signals are transmitted between at least one die and external circuitry, such as a printed circuit board, by an electrically conductive lead frame. The lead frame includes a plurality of leads, each having an inner lead end and an opposing outer lead end. The inner lead end is electrically connected to the I/O pads on the die, and the outer lead end provides a terminal for connecting to the external circuitry. Where the outer lead end terminates at a face of the package body, the package is known as a “no-lead” or “leadless” package. Examples of well-known no-lead packages include quad flat no-lead (QFN) packages, which have four sets of leads disposed around the perimeter of the bottom of a square package body, and dual flat no-lead (DFN) packages, which have two sets of leads disposed along opposite sides of the bottom of a package body.
In semiconductor device packages having an interposer type substrate, electrical signals are transmitted between at least one die and external circuitry, such as a printed circuit board, by a substrate comprising multiple layers, usually 2 or 3 thin layers, of dielectric material having electrical traces, pins, vias, and the like formed thereon. This type of substrate is typically used in grid array packages, such as ball grid array (BGA) packages, pin grid array (PGA) packages, land grid array (LGA) packages, fine ball grid array (FBGA) packages, flexible ball grid array (FxBGA) packages, and any other type of package that requires lands on the package to be arranged at a suitable circuit board-attach pitch (a “grid array”). Solder balls, bumps or pins may be disposed on the lands, as in BGA and PGA type packages, to facilitate connection to the circuit board.
In any type of molded plastic package, operation of the die generates heat that must be removed to maintain its operating integrity. While some heat is dissipated through the metallic components of the package, such as portions of the substrate and bond wires, the remainder is absorbed into the molding compound. Problematically, the molding compound is a poor thermal conductor. As a result, attempts have been made to improve the thermal dissipation performance of the molded plastic package.
One way to increase thermal dissipation performance of a molded plastic package is to dispose a metallic heat spreader within the package. In one common design, the metallic heat spreader is placed below the die. Examples of such heat spreader designs are provided, for example, in U.S. Pat. Nos. 5,367,196 and 5,608,267, both to Mahulikar et al., and in U.S. Pat. No. 5,650,663 to Parihasarathi, all of which are incorporated by reference herein in their entirety.
In another common design, the metallic heat spreader is spaced above the die, such that the die is positioned between the heat spreader and the substrate. In this design, the heat spreader typically includes downwardly directed portions that contact the substrate, the die, or both. The downwardly directed portions may be adhered to the substrate and/or die using a dielectric adhesive. Such heat spreaders typically act to spread the heat throughout the encapsulant over an area larger than that of the chip. One example of such a heat spreader is provided in U.S. Pat. No. 6,432,742 to Guan et al. Another example of such a heat spreader is provided in U.S. Pat. No. 5,977,626 to Wang et al. Unfortunately, manufacturing methods for packages employing this type of heat spreader have not lent themselves to the high level of automation needed for assembly of lower cost molded plastic packages.
One attempt to automate the assembly of molded plastic packages with heat spreaders is provided in U.S. Pat. No. 6,432,749 to Libres (the '749 patent), which is incorporated by reference herein in its entirety. The '749 patent describes a method of manufacture in which heat spreaders are provided in strip format to allow a plurality of packages to be assembled at the same time. The strip of heat spreaders is placed over the die and substrate, and a molding compound is disposed over the heat spreader, die, and substrate such that the edge of the encapsulating material is coincident with a reduced cross-sectional pillar connecting an adjacent heat spreader. The reduced cross-sectional pillar is then cut to separate the packages. Where the substrate is a leadframe, the '749 patent teaches adhering the heat spreader to the leadframe using a non-conductive adhesive to prevent electrical shorting. While this method provides for automated assembly, the assembly requires that the packages be individually molded (i.e., pocket molded) to reveal the cross-sectional pillar so that it may be cut. The use of individual molds for each package is undesirable for certain applications because it makes the molding process prone to manufacturing defects such as misalignment of the individual molds relative to the die and substrate. In addition, the method described in the '749 patent requires the use of non-conductive adhesive to secure the heat spreader to the leadframe to prevent electrical short circuits. This is another step in the manufacturing process where defects can occur.
There remains, therefore, a need for a method for the automated manufacture of thermal enhanced molded plastic packages that does not require complex manufacturing steps.